Based on the Bogoliubov approximation in the thermodynamical limit, a linear quantum many-body theory is presented to deal with the output of the bosonic atoms to a free propagating mode from a magnetic trap. It is shown that, when atoms are prepared initially in the Bose-Einstein condensation inside a trap, the output will be described by a macroscopic quantum state, a many-body coherent state. This is due to the factorization structure of the total wave function, showing that the output atom beam is coherent. The general expressions for the spectral density of the output and its spatial distribution are given in terms of our linear quantum many-body theory.